The large-size crankshaft used as the power transmission member in a marine driving source requires superior fatigue characteristics that can hardly cause fatigue breakdown even under a very severe use environment.
As a method of improving fatigue characteristics of the crankshaft, Non-patent Document 1 describes that improvement in fatigue characteristics was tried by improving fatigue characteristics with free command of processing technology. To be more concrete, the Non-patent Document 1 indicates that the RR (Roedere Ruget) method was adopted to achieve a remarkable betterment in fatigue strength as compared with a crankshaft manufactured by free forging and also that cold roll processing was applied for improvement of fatigue strength.
Non-patent Document 2 examines about possibility of improving fatigue characteristics of low-alloy steel. To be more precise, the Non-patent Document 2 mentions that (1) the inclusions in the steel are apt to become a starting point of fatigue breakdown and such inclination is likely to become more apparent as steel is developed to have higher strength; (2) the larger the size of the inclusion is, the lower the fatigue strength is likely to be; and (3) any steel material containing elongated inclusions has a tendency of easily showing anisotropy in fatigue strength.
However, the above reports do not go as far as to disclose concrete means about how to realize spheronization in morphology of inclusions and make it smaller. It is neither made clear what kinds and sizes of inclusions are to be controlled. Therefore, further studies seem to be required to realize morphology control of inclusions useful for enhancement of fatigue characteristics.
Incidentally, various methods have so far been suggested in regard to the method of morphology control for the inclusions. For example, in Patent Document 1, some suggestion is made about a method in which both sulfide and oxide are reduced in amount and inclusions are controlled in morphology as a means of producing structural low-alloy steel having excellent features in point of lamellar tear resistivity and hydrogen induced cracking resistivity. In more details, the suggestion includes that in order to suppress generation of Mn sulfide which is harmful to the lamellar tear resistivity and hydrogen induced cracking resistivity, it is effective to reduce the amount of sulfur and oxygen content while adding Ca and Mg content at the same time.
In Patent Document 2, it is described that addition of Mg and Ca works effectively to suppress growth of MnS which is apt to elongate during hot rolling and also growth of inclusions of Al2O3 and its relatives which tend to form into clusters, and further to carry out morphology control of the inclusions such as particle size refinement.
Patent Document 3 and Patent Document 4 show that by the extreme size refinement of the oxide type inclusions, it is possible to increase surface fatigue strength and gear bending fatigue strength in relation to gear material. To be more precise, these documents suggest that MgO and MgO.Al2O3, which are less coalesced, should be created as the oxide type inclusions. It is also made clear that replacing a part of sulfide MnS with (Mn.Mg)S can help suppress elongating tendency of the inclusions and reduce anisotropy of mechanical strength.
Patent Document 5 discloses that as sulfide, MnS, CaS, MgS, (Ca,Mn)S, and (Ca,Mg,Mn)S were made to exist for the purpose of obtaining machine structural steel with superior machinability. The Patent Document 5 clarifies that if the morphology of sulfide can be controlled by having REM, Ca and Mg contained, it becomes possible to suppress anisotropy of mechanical property and also to enhance the machinability to a level higher than that of S-contained free cutting steel.
However, the morphology control technology for the above inclusions are not targeting at forged parts used under such severe environmental conditions as the power transmission member in the marine driving source is subjected to. Therefore, it is now required to study and establish an original inclusion control method targeted at forging steel used for manufacturing of forged parts provided with much upgraded fatigue characteristics.    [Non-patent Document 1] “Progress and Improvement of Crankshaft,” Journal of the Marine Engineering Society of Japan, October 1973, Vol. 8, No. 10, pp. 54-59    [Non-patent Document 2] “Research on Fatigue Strength Characteristics of High-strength Crankshaft Materials,” Journal of the Marine Engineering Society of Japan, 2001, Vol. 36, No. 6, pp. 385-390    [Patent Document 1] Japanese Examined Patent Application Publication No. 58-35255 (JP-B-58-35255)    [Patent Document 2] Japanese Examined Patent Application Publication No. 57-59295 (JP-B-57-59295)    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 07-188853 (JP-A-07-188853)    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 07-238342 (JP-A-07-238342)    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2000-87179 (JP-A-2000-87179)